Petroleum distillate streams contain a variety of organic chemical components. Generally the streams are defined by their boiling ranges, which determine the composition. The processing of the streams also affects the composition. For instance, products from either catalytic cracking or thermal cracking processes contain high concentrations of olefinic materials as well as saturated (alkanes) materials and polyunsaturated materials (diolefins). Additionally, these components may be any of the various isomers of the compounds.
The composition of untreated naphtha as it comes from the crude still, or straight run naphtha, is primarily influenced by the crude source. Naphthas from paraffinic crude sources have more saturated straight chain or cyclic compounds. As a general rule most of the “sweet” (low sulfur) crudes and naphthas are paraffinic. The naphthenic crudes contain more unsaturates, cyclic, and polycylic compounds. The higher sulfur content crudes tend to be naphthenic. Treatment of the different straight run naphthas may be slightly different depending, upon their composition due to crude source.
Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction for valuable aromatic product removal. Reformed naphthas have essentially no sulfur contaminants due to the severity of their pretreatment for the process and the process itself.
Cracked naphtha, as it comes from the catalytic cracker, has a relatively high octane number as a result of the olefinic and aromatic compounds contained therein. In some cases, this fraction may contribute as much as half of the gasoline in the refinery pool together with a significant portion of the octane.
Catalytically cracked naphtha gasoline boiling range material currently forms a significant part (˜⅓) of the gasoline product pool in the United States and is the cause of the majority of the sulfur found in gasoline. These sulfur impurities may require removal in order to comply with product specifications or to ensure compliance with environmental regulations, which may be as low as 10, 20 or 50 wppm, depending upon the jurisdiction.
The most common method of removal of the sulfur compounds is by hydrodesulfurization (HDS) in which the petroleum distillate is passed over a solid particulate catalyst comprising a hydrogenation metal supported on an alumina base. Additionally, large amounts of hydrogen are included in the feed. The hydrodesulfurization reaction results in the production of hydrogen sulfide according to the following reaction: RSH+H2R′+H2S. Typical operating conditions for standard single pass fixed bed HDS reactors, such as in a trickle bed reactor, are temperatures ranging from 600° F. to 780° F., pressures ranging from 600 to 3000 psig, hydrogen recycle rates ranging from 500 to 3000 scf/bbl, and fresh hydrogen makeup ranging from 100 to 1000 scf/bbl.
After the hydrotreating is complete, the product may be fractionated or simply flashed to release the hydrogen sulfide and collect the desulfurized naphtha. In addition to supplying high octane blending components the cracked naphthas are often used as sources of olefins in other processes such as etherifications, oligomerizations, and alkylations. The conditions used to hydrotreat the naphtha fraction to remove sulfur will also saturate some of the olefinic compounds in the fraction, reducing the octane and causing a loss of source olefins. The loss of olefins by incidental hydrogenation is detrimental, reducing the octane rating of the naphtha and reducing the pool of olefins for other uses.
Various proposals have been made for removing sulfur while retaining the more desirable olefins. Because the olefins in the cracked naphtha are mainly in the low boiling fraction of these naphthas and the sulfur containing impurities tend to be concentrated in the high boiling fraction, the most common solution has been prefractionation prior to hydrotreating. The prefractionation produces a light boiling range naphtha which boils in the range of C5 to about 150° F. and a heavy boiling range naphtha which boils in the range of from about 250-475° F.
The predominant light or lower boiling sulfur compounds are mercaptans while the heavier or higher boiling compounds are thiophenes and other heterocyclic compounds. The separation by fractionation alone will not remove the mercaptans. However, in the past the mercaptans have been removed by oxidative processes involving caustic washing. A combination of oxidative removal of the mercaptans followed by fractionation and hydrotreating of the heavier fraction is disclosed in U.S. Pat. No. 5,320,742. In the oxidative removal of the mercaptans the mercaptans are converted to the corresponding disulfides.
Several U.S. patents describe the concurrent distillation and desulfurization of naphtha, including U.S. Pat. Nos. 5,597,476; 5,779,883; 6,083,378; 6,303,020; 6,416,658; 6,444,118; 6,495,030; 6,678,830 and 6,824,679. In each of these patents, the naphtha is split into two or three fractions based upon boiling point or boiling ranges.
An additional problem encountered during hydrodesulfurization is the reaction of hydrogen sulfide with olefins to form what are called recombinant mercaptans:H2S+RC═CR′RC—CR′SH+R(SH)C—CR′.The formation of mercaptans during the hydrodesulfurization of FCC gasoline is well known to occur, as disclosed in U.S. Pat. No. 2,793,170. Recombinant mercaptans may form due to the relatively high concentration of hydrogen sulfide in the flash or overhead system (compared to the concentration of hydrogen sulfide within a reactive distillation column). A very important consideration in hydrodesulfurization designs is managing the amount of these recombinant mercaptans in the product.
U.S. Pat. No. 6,409,913 discloses a process to desulfurize naphtha by reacting a naphtha feed containing sulfur compounds and olefins with hydrogen in the presence of a hydrodesulfurization catalyst. As described therein, reduced recombinant mercaptan formation may be achieved at specific conditions of high temperature, low pressure, and high treat gas ratio. Although not discussed in relation to the desired high temperature, vaporization of FCC streams may result in plugging of heat exchangers and flow lines due to the polymerization of olefins, as described in U.S. Pat. No. 4,397,739.
U.S. Pat. No. 6,416,658 a full boiling range naphtha stream is subjected to simultaneous hydrodesulfurization and splitting into a light boiling range naphtha and a heavy boiling range naphtha followed by a further hydrodesulfurization by contacting the light boiling range naphtha with hydrogen in countercurrent flow in a fixed bed of hydrodesulfurization catalyst to remove recombinant mercaptans which are formed by the reverse reaction of H2S with olefins in the naphtha during the initial hydrodesulfurization. In particular the entire recovered portion of the light naphtha from a reaction distillation column hydrodesulfurization is further contacted with hydrogen in countercurrent flow in a fixed bed of hydrodesulfurization catalyst.
Accordingly, there exists a need for processes for the hydrodesulfurization of FCC gasoline which minimizes or controls the formation of recombinant mercaptans, resulting in a hydrocarbon stream of reduced organic sulfur content.